fix(tauri-app): kill effect_update_depth_exceeded via untrack + setView

Two independent Svelte 5 effect-loop bugs that triggered the
same 'effect_update_depth_exceeded' guard:

1. PostCard.svelte: the embed-quote-fetch $effect and the
   likedBox $effect.pre read a state variable (quotedLoading /
   likedBox) and then synchronously wrote to it in the same
   effect run. Svelte 5's effect tracker schedules
   possible_effect_self_invalidation on the touched state, the
   effect re-fires immediately, and the cycle trips the
   'flush_count > 1000' guard. With ~30 PostCards mounting on
   login the per-card loop compounds into the depth exceeded
   error. Wrap the read+write blocks in untrack() so the
   hydration flags don't contribute to the effect's dep set;
   the outer 'post.embed.uri / post.did / post.rkey' reads
   remain tracked so navigation between cards still triggers a
   fresh hydrate.

2. App.svelte: the four $effect blocks (home-refresh,
   home-poll, profile-refresh, search-debounce) lived and died
   together. Even after splitting, Svelte 5 still flagged the
   call chain into refreshTimeline / refreshProfile because
   their sync prelude writes 'timelineLoading = true' /
   'profileLoading = true' while the effect already tracks the
   same downstream state via the proxy. Drive everything
   imperatively through a single setView(v) function and move
   the 5s poll into the session.subscribe callback, which fires
   only on actual login/logout transitions. setView is the
   single point that flips view AND triggers the right refresh
   per destination — NavRail on_select, LoginScreen onLogin,
   handleLogout, the 'back to timeline' button, and the tray
   navigate-event bridge all route through it now.

Also: NavRail and StatusBar self-style their grid-area in
their own component styles ('grid-area: rail' / 'status') so
App.svelte doesn't need the fragile '$state.s-XXX > nav.rail'
cross-component selector that Svelte 5 was failing to match
in the Tauri webview, leaving rail buttons invisible to clicks.

crates/tauri-app/src/lib/components/NavRail.svelte

@@ -62,7 +62,15 @@
 </nav>
 
 <style>
+  /* Self-assign the grid area so the parent App.svelte doesn't
+     need a `:global(nav.rail)` selector. Doing it via a parent
+     child-selector is fragile under Svelte 5's scoping — the
+     parent's `.shell.s-XXX > nav.rail` doesn't reliably match
+     `<nav class="rail s-YYY">` in the Tauri webview, which leaves
+     the buttons invisible to clicks. Owning the grid placement
+     here avoids the cross-component selector entirely. */
   .rail {
+    grid-area: rail;
     width: 88px;
     background: var(--bg);
     border-right: 1px solid var(--line);